It is known to use indirectly heated cathodes in the arc chambers of ion sources. In such arrangements, the cathode is formed as a button having a front thermionic electron emitting surface and a rear surface. The button is typically heated by electron impact on the rear surface, by electrons emitted and accelerated from a filament located behind the rear surface of the cathode button. With this construction, the filament is protected from sputtering by energetic particles in the arc plasma formed in the arc chamber of the ion source. The heated cathode button emits thermionic electrons at its front surface, and these are accelerated by the arc potential in the arc chamber, to initiate and maintain the required arc. The cathode button can be made relatively thick and substantial, by comparison to directly heated filament cathodes, to give the cathode longer life in operation.
An indirectly heated button cathode for an ion source is disclosed in U.S. Pat. No. 5,497,006.
Certain processes in the manufacture of semiconductor devices require the implantation of atomic species at relatively high energies, so that the species are implanted at greater depths in the semiconductor substrate. High energy ion implanters are disclosed in U.S. Pat. Nos. 4,667,111 and in 6,423,976. These prior art high energy implanters use rf linear accelerators to accelerate the ions to the high energies required for implantation. Other forms of high energy accelerators are also known for use in ion implanters, including radio frequency quadrupole (RFQ) accelerators and tandetron accelerators. Such devices have been used to produce singly charged ions of species desired for implantation at energies up to between 500 KeV and 2 MeV. Fixed voltage electrostatic accelerators are also known which can provide singly charged ions at energies in excess of 200 KeV. However, for higher energies it is known to use ions of the desired species at higher charge states, typically doubly or triply charged. The energy delivered to a charged particle by an electric field is directly proportional to the number of charges on the particle.
The operation of ion sources can be optimised to enhance the production of ions at higher charge states. However this usually involves operating the ion source with a more intense arc, so that the life of consumable elements within the ion source, particularly the cathode, is reduced. A compromise is usually made between cathode life and the beam current at the desired higher charge state.
Attempts have been made to improve the performance of ion sources in order to maximise cathode life, while operating the cathode to generate relatively high currents of desired multiply charged species. For example, “ELS2: Extended Life Source With Dual Cathode”, I. Jonoshita et al, Ion Implantation Technology—98 pp.239-241, describes a scheme using a second button cathode in the arc chamber of an ion source to replace the usual electron reflector. A modest increase in life time is demonstrated. Reference may also be made to U.S. Pat. No. 5,703,372.